Efficacy of stress-response protease-deficient Salmonella enterica serovar Typhimurium as a candidate of live oral vaccine strain

Lon蛋白酶缺陷型鼠伤寒沙门氏菌（CS2022）通过小鼠经口感染途径给药时具有免疫原性。在本研究中，单次经口接种10^7 CFU沙门氏菌后，CS2022以低于10^2 CFU持续驻留在BALB/c小鼠脾脏中长达6周。在接种后第4、6和13周，在血清中检测到鼠伤寒血清型脂多糖（LPS）特异性IgG 2a显著增加。相反，LPS特异性IgG 1在接种后6周和13周可检测到。免疫后4周和6周，小鼠脾脏IFN-γ mRNA的表达显著增加。而IL-4 mRNA的表达在接种后6周开始增加。因此，口服接种CS2022保护小鼠免受随后的皮下攻击与强毒血清型鼠伤寒。此外，当通过皮下感染途径给药时，接种能够在接种后至少6周内减少肝脏中单核细胞增生李斯特菌的定殖。此外，在接种后第2、4和6周从免疫小鼠中分离的腹腔巨噬细胞导致对单核细胞增生李斯特菌以及鼠伤寒血清型的细胞内杀伤活性增加。这些发现表明，口服免疫CS2022促进了与巨噬细胞相关的保护性先天免疫，并且这种快速的保护性免疫也可能足以在小鼠免疫后至少6周内消除除沙门氏菌以外的某些细胞内病原体的定殖。

The Lon protease-deficient Salmonella enterica serovar Typhimurium (CS2022) is immunogenic when given via the oral infection route in mice. In the present study, CS2022 continuously resided in the spleen of BALB/c mice with less than 10^2 CFU for up to 6 weeks after a single oral inoculation with 10^7 CFU of salmonellae. A significant increase in the serovar Typhimurium lipopolysaccharide (LPS)-specific IgG2a was detected in the serum at weeks 4, 6, and 13 after inoculation. In contrast, the LPS-specific IgG1 was detectable at weeks 6 and 13 after inoculation. The expression of IFN-γ mRNA in the spleens of these mice was significantly increased at weeks 4 and 6 after inoculation. In contrast, the expression of IL-4 mRNA was increased at week 6 after inoculation. As a result, oral inoculation with CS2022 protected mice against subsequent subcutaneous challenge with the virulent serovar Typhimurium. In addition, the inoculation was able to reduce the colonization of Listeria monocytogenes in the livers for at least 6 weeks after inoculation when it was administered via the subcutaneous infection route. Furthermore, peritoneal macrophages isolated from immunized mice at weeks 2, 4, and 6 after inoculation led to an increase in the intracellular killing activity against L.monocytogenes as well as against serovar Typhimurium. These findings suggest that oral immunization with CS2022 promoted the protective innate immunity associated with macrophages, and this rapid protective immunity might also be sufficient to eliminate the colonization of certain intracellular pathogens other than Salmonella for at least 6 weeks after immunization of mice.